Motor systems are multifunctional, i.e., the same peripheral structures can generate qualitatively different behaviors. Diseases of the nervous system, such as stroke, severely compromise multifunctionality. To treat such diseases, it is important to understand multifunctionality in the intact nervous system. Principles of multifunctionality could also be used to construct flexible prosthetic devices. This is the rationale for analyzing multifunctionality in the marine mollusk Aplysia californica, whose biomechanics and neural control can both be studied. Kinematic measurements in intact, behaving animals of Aplysia's feeding muscles using magnetic resonance imaging have clarified the biomechanical basis of multifunctionality. The kinematics of the muscles, and in vivo recordings of neural activity correlated with the kinematics, indicate how activity in motor pools controlling buccal mass muscles must be altered to generate different feeding responses. Furthermore, these data suggest that multifunctionality of feeding in Aplysia is mediated by differential activation of context dependent identified interneurons that cause some of the observed shifts in timing, intensity and duration of the motor neuronal pools controlling muscles of the buccal mass to generate appropriate movements for qualitatively different feeding responses. To test this hypothesis, three Specific Aims will be pursued. First, we will simultaneously characterize the in vivo activity of interneurons and selected motor neurons that control the buccal muscles. Second, we will study reduced preparations capable of generating feeding motor patterns, and examine the effect of hyperpolarizing or depolarizing interneurons on the resulting motor patterns. Third, we will test the hypothesis in vivo using a novel electrophysiological technique that will allow us to turn on or turn off the activity of different interneurons in intact, behaving animals. These studies will lead to a deeper understanding of the neuromechanical principles underlying multifunctionality.